modern industry can not be imagined without a material such as steel.With it, we are confronted almost every turn.By the introduction into its composition of different chemical elements can significantly improve the mechanical properties and performance.What
steel
became known as an alloy that is composed of carbon and iron.Also, such an alloy (photo disposed below) may have impurities of other chemical elements.
There are several structural states.If the carbon content is within 0,025-0,8%, the data called pro-eutectoid steel and have in their structure perlite and ferrite.If hypereutectoid steel, it is possible to observe the pearlite and cementite phase.A feature of the ferritic structure is the most plasticity.Cementite also has a considerable hardness.Perlite form two previous phases.It can have a granular shape (by inclusion of ferrite grains arranged cementite which have a circular shape) and the plate (both phases have the form of plates).If the steel is heated above the temperature at which the polymorphs, the structure changes to austenite.This phase has a high plasticity.If the carbon content exceeds 2.14%, such materials and alloys are called iron.
steels
Depending on the composition of the steel may be a carbon and alloy.The carbon content less than 0.25% carbon steel is characterized.When the amount reaches 0.55%, then we can talk about the medium-alloy.Steel, which is composed of more than 0.6% carbon, called high-carbon.If the way is made alloy technology involves the introduction of specific chemical elements, this steel alloy called.Introduction of various components significantly change its properties.If the number does not exceed 4%, the low-alloy alloy.Srednelegirovannoj and high-alloy steel is respectively 11% and more than 12% impurities.Depending on in which area of steel alloys are used, they are isolated species: instrumental, construction and special steels and alloys.
Technology of
steelmaking process is quite time consuming.It includes several stages.First of all, you need raw materials - iron ore.The first stage comprises heating to a certain temperature.This oxidative processes occur.In the second stage the temperature is considerably higher.Oxidation of carbon are more intense.Perhaps additional oxygen enrichment of the alloy.Unwanted impurities are removed to the slag.The next step is intended to remove oxygen from steel, because it significantly reduces the mechanical properties.This can be carried out or precipitating a diffusion method.If the process of deoxidation occurs, the resulting steel is called boiling.Calm alloy does not emit gases, oxygen is removed completely.An intermediate position is occupied balanced steel.Production of alloys of iron occurs in the open hearth, induction furnaces, oxygen converters.
alloying
In order to obtain these or other properties of steel, its composition is administered special dopants.The main advantages of this alloy are increased resistance to various strains, the reliability of parts and other structural elements is greatly increased.Quenching reduces the percentage of cracks and other defects.Often such different elements saturation method is used to impart resistance to chemical corrosion.But there are some drawbacks.They require additional processing, a high probability of occurrence of hairline cracks.Besides the cost of material increases.The most common alloying elements - chromium, nickel, tungsten, molybdenum and cobalt.Their field of application is quite large.This engineering, and production of pipeline, power, aviation and many more.
concept of high-temperature strength and heat resistance
The term heat resistance means the ability of a metal or alloy to keep all its characteristics when working in high temperatures.In such an environment it is often observed gas corrosion.Therefore, the material must possess resistance to its action, that is to be temperature-resistant.Thus, the characteristics of alloys are used in large temperature should include both concepts.Only then such steel will provide the necessary resources for the operation of parts, tools and other structural elements.
Features heat-resistant steel
When the temperature reaches high values, requires the use of alloys that will not break down and give in to the strain.In this case, use a heat-resistant alloys.The operating temperature for such materials - above 500 ° C.An important point with similar steel is a high endurance limit, plasticity, which persists for a long time, as well as the relaxation resistance.There are a number of elements that can greatly enhance the resistance to high temperatures: cobalt, tungsten, molybdenum.It is a compulsory component and chromium.He does not so much affect the strength, it increases the resistance to scaling.Chrome also prevents corrosion processes.Another important characteristic of this type of alloys - a slow creep.
Classification of heat-resistant steels for the structure
Heat-resistant and heat-resistant alloys are ferritic class, martensitic, austenitic and martensitic-feritno.The first are composed of about 30% chromium.After special treatment structure becomes fine.If the heating temperature exceeds 850ºS, the grain increases, and such heat-resistant materials become brittle.Martensitic grades characterized by a chromium content ranging from 4% to 12%.Also may be present minor amounts of nickel, tungsten and other elements.Of them produce parts of turbines, valves in cars.Steels which have in its structure a martensite and ferrite, suitable for operation at high temperatures and constant long-term operation.The chromium content reaches 14%.Austenite is obtained when administered in nickel superalloys.Steel with such a structure have a number of brands.
Nickel-based alloys
Nickel has a number of useful properties.It has a positive effect on the machinability of the steel (either hot or cold).If the item or a tool designed to work in a hostile environment, the doping of this element significantly increases the resistance against corrosion.Heat-resistant nickel-base materials are separated into the following groups: heat resistant and heat resistant itself.The latter should also have a minimum heat-resistant characteristics.Operating temperatures reach 1200ºS.Additionally introduced chrome or titanium.Characteristically, steel, nickel alloy, have a slight amount of impurities, such as barium, magnesium, boron, therefore, more grain boundaries are reinforced.Superalloys of this type are available in the form of forgings and rolled.It is also possible ebb details.The main area of application - the production of gas turbine components.Heat-resistant nickel-based alloys are composed of up to 30% chromium.They are quite well to punching, welding.In addition, scaling resistance is high.This makes it possible to use them in the gas pipeline system.
Heat-resistant steel, alloy titanium
Titanium is introduced in a small amount (0.3%).In this case it increases the strength of the alloy.If its content is considerably higher, some deteriorate mechanical properties (hardness, strength).But at the same time increases the plasticity.This facilitates the handling of steel.When administered to titan together with other components can significantly improve high-temperature characteristics.If there is the need to work in a hostile environment (especially in the case when the construction involves welding), the alloying element chemically data is warranted.
cobalt alloys
large amount of cobalt (80%) goes to the production of materials such as heat-resistant and heat-resistant alloys, as in pure form, it is rarely used.Its administration improves ductility and durability under high temperatures.And the higher it is, the higher the amount of cobalt incorporated in the alloy.In some stamp its content reaches 30%.Another characteristic of these steels - improvement of magnetic properties.However, due to the high cost of cobalt, its use is fairly limited.
Effect of molybdenum on superalloys
This chemical element significantly affects the strength of the material at high temperatures.Especially effective is its use in conjunction with other elements.It significantly increases the hardness of the steel (already at a content of 0.3%).Tensile strength also increases.Another positive feature of which are heat-resistant alloys doped molybdenum - a greater degree of resistance to oxidation processes.Molybdenum contributes to grain refinement.The disadvantage is the difficulty of welding.
Other special steels and alloys
to perform various tasks required materials that have certain properties.Thus, we can talk about the use of special alloys which can be either doped or carbon.The final set of required characteristics is attained due to the fact that the production of alloys and their processing is for special technology.Another special steels and alloys are divided into structural and instrumental.Among the main tasks for this type of materials are the following: resistance to corrosion and wear, the ability to work in a hostile environment, improved mechanical properties.In this category, and are heat-resistant steels and alloys with a high operating temperature, and cryogenic steel that can withstand up to -296ºS.
Tool steel for tools used in the production of special tool steel.Given that the working conditions of different materials are also selected individually.Since the requirements for the instruments are high enough, then the characteristics of the alloy to produce them accordingly: they must be free of external impurities, inclusions, deoxidation process is well organized and homogeneous structure.For the instrumentation it is very important to have a stable performance and resist wear.If we talk about cutting tools, they operate at high temperatures (heat holds the edge), constant friction and deformation.So for them it is very important to keep the primary hardness when heated.Another type of tool steel - high speed.In general, it is doped tungsten.Hardness is maintained at a temperature of about 600ºS.There are also die steels.They are designed for both hot and cold deformation.
Scope alloys Special
Industries that use alloys with special characteristics set.Because of its superior quality, they are indispensable in the engineering, construction, oil industry.Heat-resistant and heat-resistant alloys are used in the manufacture of turbine parts, spare parts for cars.Steels which have a high anticorrosive characteristics are essential for the production of tubes, needles, carburetors, wheels, all kinds of chemical elements.The rails for the railroad, buckets, caterpillars for transport - the basis for all this is the wear-resistant steel.In mass production of bolts, nuts and similar parts used alloys automaton.Suspension must be sufficiently resilient and durable.Therefore, the material to them is a spring steel.To improve the quality of this they additionally alloyed with chromium, molybdenum.All special alloys and steels with a set of specific characteristics allow to reduce the cost of parts where previously used non-ferrous metals.
